Protection devices are provided as, among others, lightning and/or overvoltage protection in the primary power system of physical structures, wherein the external conductors are each connected to the neutral conductor by means of a lightning surge arrestor. Such a device is also known as a discharge arrangement or surge arrestor arrangement.
An arrestor serves to prevent hazards to life, system and device damage, and fire. The task of the lightning and overvoltage protection is the reduction of overvoltages to a protection level of 1.5 kV, and arresting strong current surges up to 50 kA. Explosive flames and shock waves must be avoided in order to enable installation in typical equipment without observing special safety clearances and passive fire prevention measures. The extinguishing capabilities and the limiting of the power frequency follow current must be designed so that service line main fuses are not triggered. Finally, compatibility with standardized assembly systems and ruggedness with respect to climate influences and pollution are required.
Typical characteristic data of such lightning and overvoltage protection devices are: a D.C. spark-over voltage of Vsdc>600 V, a surge spark-over voltage vas≦1.5 kV for a voltage increase rate of 5 kV/μs, a nominal surge current iSN of 50 kA for standardized surge current curves of 8/20 μs and 10/350 μs and, in each case, 15 times load factor, and a power frequency follow current of ≦50 kA.
When a surge arrestor is triggered, an arc is generated that creates combustion products, which can precipitate as a conductive vaporization on the interior cylinder of the ceramic or insulating material and cause a fault in the insulation.
For fulfilling the protection function, a low dynamic spark-over voltage is required. For the entire component the required value is 1500 V, if a ramp-shaped voltage increasing at a rate of 5 kV/μs is applied. For a single arrestor, this level of protection and the remaining characteristic data imply that an ignition aid must be used. One way to improve the ignition behavior is the introduction of an ignition strip made of graphite, for example, which makes primary charge carriers available, biases the electrical field, and improves the dynamic ignition behavior by means of a creeping discharge along the ceramic surface of the inner wall of the cylinder.